there's hundreds of us-- hundreds, thousands of us. look, 31,000 scientists and engineers signed a statement to the contrary to what you just read. >> hockenberry: the oregon petition? >> yes. >> hockenberry: the 14-year-old petition is not exactly an exclusive club. a bachelor of science degree is all it takes to get you on the list. this document, skeptics claim, counters the scientific consensus on global warming. now, are they all scientists? >> yes. one-third of them have ph.d.s. look, they are not specialists in climate. >> hockenberry: well, some were celebrities and friends... >> eh... >> hockenberry: it's a time- honored tactic by the skeptics: authentic-looking documents and reports that don't stand up to independent scrutiny. singer also signed the oregon petition. this is not his first time going up against accepted science. >> hockenberry: was the science around chlorofluorocarbons hyped, the science around secondhand smoke hyped, the science around the ozone layer hyped, going back ten, 15, 20 years? >> i'm happy to discu